Hughes et al. Nature 258, 557 (1975), have described the isolation of a pentapeptide, H-Tyr-Gly-Gly-Phe-Met-OH- from pig brains. The pentapeptide exhibits opiate agonist activity.
This amino acid sequence occurs as residues 61 to 65 of ovine beta-lipotropin. See the papers by Li and coworkers, Nature 208, 1093 (1965); Excerpta Medica Internatl. Congr. Ser. 112, 349 (1966); and Biochem. Biophys. Res. Commun. 53, 1304 (1973).
Peptide-like substances having morphine like activity derived from bovine and porcine pituitaries have been reported by Goldstein and coworkers. Life Sciences, 16, 1775 (1975). Hughes et al. have suggested that these substances may be the peptide fragments derived from enzymatic digests of beta-lipoprotein.